Optical device and method for the non-intrusive measuring of the temperature of a flowing liquid

ABSTRACT

An optical method, and an optical device based on a laser source for implementing an optical method, for nonintrusively measuring the temperature of a flowing liquid by using the fluorescence induced by a laser beam in a measurement volume of the liquid, uses a single temperature-sensitive fluorescent tracer and at least two separate spectral detection windows on the tracer, after molecular dilution of the tracer in the liquid medium.

BACKGROUND OF THE INVENTION

The present invention relates to an optical method and to an opticaldevice which uses a laser source for nonintrusively measuring thetemperature in a flowing liquid.

Temperature measurement devices based on a laser source are known. Forexample, European Patent Application No. 345188 discloses a device whichoperates based on the observation of interference fringes created withina flowing fluid in a pipe. These observations are made at the output oftwo optical fibers which transport light from an external laser source.The disclosed apparatus is satisfactory for such situations, but doesrequire intrusion in the fluid being studied, something that is notalways possible.

Various nonintrusive techniques for measuring the parameters of a fluid(for example, temperature or pressure) are already known. For example,French Patent Application 2,579,320 discloses an ultrasonic wavemeasurement method which is intended for the thermal monitoring ofnuclear reactors. The disclosed technique allows only a single parameterof the fluid to be measured, namely, its temperature.

The main drawback of presently known nonintrusive temperaturemeasurement devices is their excessively long response time. Indifficult situations, such as measurement of the temperature ofdroplets, in a spray for example, where the measured volume changes dueto evaporation over time during measurement, in liquid-gas two-phaseflow, in the measurement of the temperature field in a plane, and in themeasurement of temperature in a turbulent flow, such measurement remainsdifficult, if not impossible to carry out. Also, there are difficultiesin obtaining an apparatus that has a short response time and whichdisturbs the flow as little as possible.

SUMMARY OF THE INVENTION

To solve these problems, it has been found that optical methods can beused. In particular, it has been found that laser-induced fluorescencemethods useful for determining the concentration of a fluid can be used.Such methods operate according to the principle which follows.

Fluorescence, a physical phenomenon which has long been known, is theconsequence of an excited state of a fluorescent species beingdeactivated toward a ground state by spontaneous emission. The excitedstate may be induced by laser radiation having a wavelength whichcoincides with the absorption spectrum of the fluorescent species. Thetime between the absorption and emission of a photon is of the order ofa few nanoseconds, which makes the technique applicable to the study ofrapidly varying phenomena. The time resolution may reach a few tens ofkHz.

When laser radiation passes through a medium seeded with a lowconcentration (or a higher, but constant or slightly varyingconcentration) of a fluorescent tracer, the fluorescence intensity maybe expressed by the equation:I _(fluo) =K _(opt) K _(spec) CV _(c) I _(o) e ^(β) ¹ ^(/T)where K_(opt), V_(c), I_(o) and C are the coefficients characterizingthe optical system, the volume in which the fluorescent photons arecollected, the incident laser intensity, and the molecular concentrationof the fluorescent tracer, respectively, and I is the temperature.K_(spec) and β are constants, depending only on the characteristics ofthe molecule used as the fluorescent tracer.

Applying this principle allows the temperature to be obtained in simplesituations in which the volume, the concentration and the local laserintensity are well controlled or remain constant.

A first technique for applying this physical principle to themeasurement of temperatures partly eliminates the drawbacks previouslymentioned by using two fluorescent tracers to solve the problems oflaser intensity and volume. However, many difficulties neverthelessremain. The concentration of each of the two tracers must be controlled,and the emission spectra of the two tracers must be sufficientlyseparate so as to be able to separate the fluorescent emissions of eachof the tracers by means of a set of interference filters. However, thisis difficult to achieve in practice.

To resolve all of the above-listed problems, it has been found that asingle tracer, with molecular dilution in the liquid medium whosetemperature is to be measured, can be used.

Moreover, U.S. Pat. No. 5,788,374 A refers to chemiluminescence inducedby white light (lamp or xenon). The photophysics used is completelydifferent from that of the present invention, and in actuality relies onthe detection of fluorescence in two spectral bands, but from twodifferent products created by a chemical reaction, the kinetics of whichdepend on temperature.

The reaction disclosed in U.S. Pat. No. 5,788,374 A is of the type,M+hυM*. In the presence of a partner N (which in the cited U.S. patentappears to be the matrix of the molten polymer in which the fluorescentproduct is in solution), the monomer in the excited state M* can eitheremit fluorescence in a wavelength range 1M*→M+hυ₁ or be converted intoan exciplex E* after reacting with the partner N, which itself will giveoff fluorescence in a wavelength range according to the reaction:M*+N→E*E*→E+hυ ₁The ratio of the fluorescence of the monomer, in a first wavelengthrange, to that of the exciplex, in a second wavelength range whichdepends on temperature, is therefore detected.

In the foregoing technique there are, therefore, two fluorescentproducts. In the two-color fluorescence technique of the presentinvention, there is only a single fluorescent product. In addition, thetwo-color fluorescence method of the present invention uses ultrarapidphotophysics principles relating to electronic transition andcollisional deactivation phenomena, and not the kinetics of a chemicalreaction induced by irradiation. The response time of the technique isin this way considerably improved.

Also known is French Patent Application No. 2,484,639, which discloses amethod which is an intrusive method because it requires the presence ofsensors at various measurement locations, whereas the method of thepresent invention is a nonintrusive method. Furthermore, the fluorescentmaterial used is a solid-state material (see, page 8, lines 2–3),whereas the tracer of the present invention is a tracer in molecularsolution in the liquid whose temperature is being measured.

Also known, from a Japanese Patent Abstract, is a method which seedswith fluorescent particles, and not with a tracer in molecular solution,and which uses irradiation with UV light, and not a laser radiation.

The present invention employs an optical method for nonintrusivelymeasuring the temperature of a flowing liquid which uses fluorescenceinduced by laser radiation in a measurement volume of the liquid, andwhich uses a single temperature-sensitive fluorescent tracer and atleast two separate spectral detection windows on this same tracer, aftermolecular dilution of the tracer in the liquid medium.

The method preferably comprises the steps of receiving an optical signaland completely eliminating scattering or reflection of the excitinglaser component, splitting the optical signal into several lightsignals, creating a detection window for each light signal obtainedafter the splitting step, amplifying the light signals received in thedetection windows and converting the light signals into a correspondingnumber of electrical signals, and acquiring, processing and displayingthe electrical signals.

A device for nonintrusively measuring the temperature of a flowingliquid, in accordance with the present invention, by using thefluorescence induced by laser radiation in a measurement volume of theliquid, is adapted to implement the method of the present invention andcomprises a single reception channel having a holographic band-rejectionfilter and a unit for splitting the optical signal into two lightsignals, at least one optical measurement channel having a filter forcreating a measurement window and an amplifier for amplifying andconverting the light signals into electrical signals, and a computingsystem.

Preferably, the laser radiation is produced by a single laser beam, adouble laser beam, or a laser sheet, and the filter creating themeasurement window is a band-pass interference filter, a high-passfilter, or a low-pass filter. Preferably, a band-pass interferencefilter having a bandwidth Δλ₁, through which a first signal passes,centered on a wavelength λ₁, is chosen for one measurement channel and ahigh-pass filter, through which a second signal passes, letting throughan optical intensity above a threshold wavelength λ₂, is chosen for theother measurement channel.

A further understanding of the present invention will be gained from thedescription of a nonlimiting embodiment which is provided below, withreference to the following illustrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the operating principle of the device of thepresent invention.

FIG. 2 is a graph showing the positions of the detection windows on theemission spectrum of Rhodamine B.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention uses a single fluorescent tracer.The tracer is in molecular dilution in the liquid medium whosetemperature is to be measured, there is no transformation of thechemical type, and the fluorescent properties of the tracer arise fromone and the same molecule.

In a nonlimiting example, which is described below, the tracer used isRhodamine B (C₂₈H₃₁ClN₂O₃), which is known to be particularly sensitiveto temperature. In addition, the temperature sensitivity of this tracerdiffers depending on the spectral band of the fluorescent spectrum inquestion. In a first example, the present invention uses a single tracerand two separate spectral detection windows on this same tracer. Theresulting sensitivity of the method of the present invention is around2% of the variation in the fluorescence signal per ° C., which, takinginto account the observed signal-to-noise ratio, leads to an accuracy towithin 1° C. in the temperature. Detection therefore takes place on twopredefined spectral windows, and not by studying complete spectra. As aresult, the use of a spectrometer is not required. Instead, simplephotodetectors can be used, for example, photodiodes, photomultipliers,CCD cameras, etc.

To implement this method, and referring to FIG. 1, a device (1) isprovided which is based on the fluorescence induced by laser radiation(2) in a measurement volume (3) of a flowing liquid, allowing splittingof the signals and the detection thereof. The laser radiation can be inthe form of one or two laser beams, or a laser sheet. In particular, theuse of two laser beams intersecting at a point defining the measurementvolume makes it possible to simultaneously measure the velocity using acommercially available velocity meter system.

As an example, the device comprises a single reception channel with aholographic band-rejection filter (6) for removing all scattering orreflection of the exciting laser component, and a unit (4) for splittingthe optical signal (5) into at least two equal or non-equal lightsignals (5 a, 5 b). For example, two light signals can be defined, usingtwo neutral splitters (4 a, 4 b), according to the desired power in eachchannel.

In each optical measurement channel, a filter, possibly a band-passinterference filter, a high-pass filter or a low-pass filter, is used tocreate a measurement window appropriate to the tracer used. For theexample described herein (Rhodamine B dissolved in ethanol), thefollowing was preferably selected for use. The signal (5 a) passesthrough a band-pass interference filter (7), making it possible toobtain the first measurement window (10) shown in the graph of FIG. 2.For the tracer used, the filter (7) is centered on a wavelength λ₁±Δλ₁,preferably 530 nm±5 nm. The signal (5 b) passes through a high-passfilter (8), letting through the optical intensity above a thresholdwavelength λ₂; preferably λ₂=approximately 590 nm. The essentialcriterion in the choice of wavelengths is that of having differenttemperature sensitivities.

The windows and the wavelengths are chosen according to the temperatureresponse curve of the tracer or of the tracer-solvent mixture. Costcriteria may also be involved in the choice of filters. In the presentdescription, and in the claims which follow, the term “window” isunderstood to have its general meaning, i.e., a window possibly beingcomposed of one or more wavelength ranges, which may or may not belimited. Likewise, the term “separate” does not exclude any partialoverlap or the inclusion of the windows, and the expressions“measurement window” and “detection window” have the same meaning.

An amplifier (9 a, 9 b) is provided for amplifying and converting thelight signals (5 a, 5 b) into electrical signals, for example, aphotomultiplier, a photodiode or a CCD sensor. Such amplifiers providefor the precise measurement, with a short response time, of thefluorescence intensities. A computing system (11), with correspondingsoftware, receives the electrical signals via an acquisition system (notshown), processes the received electrical signals, and displays theresults on a screen.

The collected fluorescence intensity of the first signal (5 a) can bewritten according to the simplified equation:I _(f1) =K _(opt1) K _(spect1) CI _(o) V _(c) e ^(β) ¹ ^(/T).Likewise, the fluorescence intensity of the second signal (5 b)satisfies the equation:I _(f2) =K _(opt2) K _(spect2) CI _(o) V _(c) e ^(β) ² ^(/T).

Both of the above equations are expressed with the proviso that therelevant absorption coefficients with respect to the fluorescenceintensity are similar in the relevant spectral windows, or that theproduct C·x can vary slightly in relation to the control, therebyallowing simplification, where x denotes the optical path traveled bythe fluorescence signal in the absorbent medium.

Thus, the ratio of the measurement of the fluorescence intensity of thesignals received in the detection windows is given by the equation:

$\begin{matrix}{R_{f} = {\frac{I_{f1}}{I_{f2}} = {Ke}^{{({\beta_{1} - \beta_{2}})}/T}}} & \left( E_{1} \right)\end{matrix}$where K is a constant that only depends on the optical system used, andon the spectroscopic properties of the molecule used as the fluorescenttracer. This ratio has the advantage of being independent of theconcentration of the fluorescent tracer, of the exciting laserintensity, and of the excited fluorescent volume. The apparatus constant(K) is determined by a single initial calibration point at a knowntemperature T_(o). Equation (E₁) is thus transformed to:

$\begin{matrix}{{\ln\mspace{11mu}\left( \frac{R_{f}}{R_{fo}} \right)} = {\left( {\beta_{1} - \beta_{2}} \right)\mspace{11mu}\left( {\frac{1}{T} - \frac{1}{T_{o}}} \right)}} & \left( E_{2} \right)\end{matrix}$where R_(fo) is the ratio at the temperature T_(o). This law wasconfirmed experimentally.

In addition to the temperature, it is possible to simultaneously obtainthe concentration of the liquid, or the measurement volume (3).

When the relevant absorption coefficients with respect to thefluorescence intensity are not similar in the relevant spectral windows,or when the product C·x varies significantly with respect to thecontrol, the absorption phenomena of the fluorescence must be taken intoaccount. In this case, a third channel and a third measurement windowcan be added so as to quantify this absorption.

The tracer chosen was Rhodamine B, diluted in alcohol to 2 mg/l, whichhas been found to be satisfactory and to allow two spectral bands to beisolated. However, other tracers, such as, for example, Oregon 488,Rhodamine 6 G, and other dilution liquids such as water, could be used.

The present invention can advantageously be used to measure thetemperature of spray droplets. The difficulty in using prior techniquesresides in the variation in the measurement volume because of thetemperature. Since this parameter is absent from equation E₂, it has noinfluence on the measurements obtained in accordance with the presentinvention. The present invention can also be used to measure thetemperature in a turbulent flow, for which devices with a suitableresponse time are unknown. The present invention can also be used tomeasure the temperature distribution in a measurement plane throughwhich a flowing fluid passes, independently of the variations in thelocal laser intensity.

1. An optical method for nonintrusively measuring temperature of aflowing liquid, comprising the steps of: placing a singletemperature-sensitive fluorescent tracer in molecular dilution in theliquid in a defined measurement volume; defining at least two separatespectral detection windows on the single temperature-sensitivefluorescent tracer; inducing fluorescence in the measurement volumeusing laser radiation; detecting the fluorescence induced in themeasurement volume in the separate spectral detection windows andprocessing the detected fluorescence induced in the measurement volumein the separate spectral detection windows to measure the temperature ofthe flowing liquid.
 2. The method of claim 1 wherein the tracer isRhodamine B (C₂₈H₃₁ClN₂O₃).
 3. The method of claim 1 wherein thedetecting and processing step further includes the steps of: receivingan optical signal corresponding to the fluorescence induced in themeasurement volume, and completely eliminating scattering and reflectionof excited laser components; splitting the optical signal into aplurality of light signals; defining separate measurement windows foreach of the light signals obtained after the splitting; amplifying eachof the light signals received in the separate measurement windows, andconverting the light signals into a corresponding number of electricalsignals; and acquiring, processing and displaying the electricalsignals.
 4. The method of claim 3 which uses two measurement windows,and which further includes the steps of measuring fluorescence intensityfor the light signals received in the two measurement windows, andcalculating a ratio Rf of a first measured fluorescence intensity I_(f1)for the light signals received in a first of the two measurement windowsand a second measured fluorescence intensity I_(f2) for the lightsignals received in a second of the two measurement windows, wherein:$R_{f} = {\frac{I_{f1}}{I_{f2}} = {{Ke}^{{({\beta_{1} - \beta_{2}})}/T}.}}$where K is a constant which depends on the fluorescence intensitymeasurement and on spectroscopic properties of the tracer, β is aconstant which depends on characteristics of the tracer, and T is thetemperature.
 5. The method of claim 4 which further includes the step ofdetermining the constant K at a single initial calibration point at aknown temperature To, and transforming the ratio to an equation:${{\ln\left( \frac{R_{f}}{R_{f\; o}} \right)} = {\left( {\beta_{1} - \beta_{2}} \right)\left( {\frac{1}{T} - \frac{1}{T_{o}}} \right)}},$where R_(fo) is the ratio at the temperature T_(o).
 6. The method ofclaim 1 which further includes the step of simultaneously measuring anadditional characteristic selected from the group of characteristicsconsisting essentially of a velocity of the liquid, a concentration ofthe liquid, and the measurement volume.
 7. A device for nonintrusivelymeasuring temperature of a flowing liquid using the optical method ofclaim 5, the device comprising: a single reception channel including aholographic band-rejection filter and a splitter for splitting anoptical signal corresponding to the fluorescence induced in themeasurement volume into two light signals; an optical measurementchannel for receiving each of the two light signals, wherein eachoptical measurement channel includes a filter for defining a measurementwindow, and an amplifier for amplifying and converting the light signalsinto electrical signals; and a computing system for processing theelectrical signals.
 8. The device of claim 7 wherein the filter fordefining the measurement window is selected from the group of filtersconsisting essentially of a band-pass interference filter, a high-passfilter, and a low-pass filter.
 9. The device of claim 8 wherein a firstoptical measurement channel includes a band-pass interference filterhaving a bandwidth Δλ₁ centered on a wavelength λ₁, for receiving afirst of the light signals, and a second optical measurement channelincludes a high-pass filter passing optical intensities above athreshold wavelength λ₂, for receiving a second of the light signals.10. The device of claim 9 wherein λ₁ is equal to approximately 530 nm.11. The device of claim 9 wherein λ₂ is equal to approximately 590 nm.12. The device of claim 9 which further includes an additionalmeasurement window and an additional measurement channel for quantifyingabsorption phenomena of the fluorescence induced in the measurementvolume.
 13. The device of claim 7 wherein the laser radiation isproduced by a laser source selected from the group of sources consistingessentially of a single laser beam, a double laser beam, and a lasersheet.
 14. The device of claim 7 which further includes photodetectorsfor detecting the light signals.